Ultraslow calorimetric studies of the martensitic transformation of NiFeGa alloys: detection and analysis of avalanche phenomena

Abstract

AbstractWe study the thermal properties of a bulk $$\hbox {Ni}_{55}\hbox {Fe}_{19}\hbox {Ga}_{26}$$ Ni 55 Fe 19 Ga 26 Heusler alloy in a conduction calorimeter. At slow heating and cooling rates ($$\sim {1}\,\hbox {K h}^{-1}$$ ∼ 1 K h - 1 ), we compare as-cast and annealed samples. We report a smaller thermal hysteresis after the thermal treatment due to the stabilization of the 14 M modulated structure in the martensite phase. In ultraslow experiments ($${40}\,\hbox {mK h}^{-1}$$ 40 mK h - 1 ), we detect and analyze the calorimetric avalanches associated with the direct and reverse martensitic transformation from cubic to 14 M phase. This reveals a distribution of events characterized by a power law with exponential cutoff $$p(u)\propto u^{-\varepsilon }\exp (-u/\xi )$$ p ( u ) ∝ u - ε exp ( - u / ξ ) where $$\varepsilon \sim 2$$ ε ∼ 2 and damping energies $$\xi ={370}{\upmu {\rm J}}$$ ξ = 370 μ J (direct) and $$\xi ={27}{\upmu {\rm J}}$$ ξ = 27 μ J (reverse) that characterize the asymmetry of the transformation.